Optical transmission elements such as optical cables or optical cores, f.e. in the form of so-called bundles, in general contain optical waveguides which are surrounded by a slot element enclosing them. A usual method for anchoring the optical waveguides in an optical transmission element is filling the slot elements with high-viscose, thixotropic or cross-linked filling compound. Such a filling compound has the disadvantage, that it can leak out or drip out in case of vertically hanging ends of the transmission element. Additionally, in case of splitting the transmission element during installation, leaking filling compound can lead to contamination and handling problems on the part of the installation personnel.
Such a filling of the optical transmission element has the advantage, that during the manufacturing process of the optical transmission element during the extrusion of the slot element in the form of a tube, the still soft cladding of the tube is supported by the filling compound and the round cross-section remains stable until curing occurs. Furthermore, water, which penetrates due to damage to the transmission element into the tube, is prevented by the filling compound from further penetration.
Water tightness of the transmission element can also be produced, when the slot tube of the transmission element is only filled with filling compound in sections or with several dry and compressible filling elements, which provides the additional advantage, that costs of filling compound can be reduced. However, a disadvantage of this sectional filling of the transmission element is a discontinuation of the support effect for the still soft slot tube immediately after extrusion. Without additional procedures, this can lead to sections of non-round slot tubes, which at least complicate or prevent a symmetrical cable construction or core construction, respectively.
In order to get a round cross-section of an optical transmission element without continuous filling compound, so-called outer calibration can be used as an additional procedure. With this, the soft slot tube is pulled through a slot, where there is a vacuum relative to the environment. This vacuum sucks the slot walls towards the outside against the inner contours of so-called disc tips, past which they are pulled and then cured. Such outer calibrations devices are generally technically difficult to produce and have to be calibrated for each outer diameter of a transmission element.